In recent times, there has been much interest in the microwave heating and drying of fabrics. Some of the major advantages of the microwave drying of fabrics is the ability to sanitize the clothes, as illustrated in U.S. Pat. Nos. 4,896,010 and 4,829,679.
Another major advantage of microwave drying is the substantial reduction of lint and fabric wear which results from the speed of the drying process.
Through the years, a major drawback to the utilization of microwave heating and drying of fabrics has been the problem of arcing from large metal objects; this often results in the scorching and burning of fabric.
Recent progress has been achieved in preventing substantial arcing which is caused by permanent fabric fasteners, such as metal buttons, metal rivets and metal zippers. However, it is still problematical when drying fabrics on which small foreign metal objects (such as paper clips, hair pins, twist-ties with wire, etc.) are found. These small objects are often left in clothes pockets and are sometimes found in cuffs and folds of fabric.
Not only is the presence of small metal objects more difficult to ascertain, but paper clips and hair pins also present a different arcing problem than larger pieces of metal.
Paper clips, hair pins and twist-ties tend to resonate when subjected to high-powered microwave radiation, which produces voltages and currents sufficient to melt them. This condition, in turn, can cause scorching and burning of the fabrics in which they are imbedded.
Obviously, written warnings to inspect clothing and fabric are insufficient to protect against this hazard.
The resonant frequencies emitted by these small objects during microwave heating cannot be successfully detected. It was found that the emissions from these objects produced wide band amplitude modulated noise, with no specific frequency signature. What signals were detected were often in the low pico-watt range. Some of the emitted radiation was believed to be reabsorbed and converted to heat. The main factor causing lack of detection was determined to be the small size of these objects. A small "antenna" length is not sufficient to provide a reliably detectable electromagnetic signal.
The current invention has determined that these small objects can be detected by a "UVtron" device. A "UVtron" is a commercially available ultraviolet light detector tube, manufactured by Hamamatsu. The ultraviolet light detector tube operates in similar fashion to a phototube, wherein an ion chamber is biased in the discharge or avalanche region.
Arcs from resonant loads of small metal objects produce intense electromagnetic radiation between 100 and 800 nanometers. With or without appropriate selected range cutoff for ambient light triggering, the UV tube of this invention is able to reliably detect small metal objects subjected to microwave radiation. The detected signal is then used to terminate the drying operation.
In conjunction with the above detection apparatus, the invention also discovered that a resonant object in contact with moist or wet fabric tended not to arc as readily as when it was in contact with dry fabric. It was, therefore, determined that a possible arcing condition could be prevented by varying the power output of the magnetron as a function of time or as a function of sensed dryness. In other words, the power was reduced or terminated toward the end of the drying cycle, in order to reduce the possibility of arcing. The power can be cut off in the last few minutes of the microwave drying cycle, and the clothing brought to drying completion through normal hot air convection techniques, i.e., by using electrical radiant heating.
The "end of heating cycle" dryness condition can be sensed in a number of ways consistent with the teachings illustrated in U.S. Pat. No. 4,795,871.